Facebook's Messenger app for iPhone was updated to version 4.0 earlier today, adding support for group creation and a "Forward" feature for individual messages reminiscent of the same option in Apple's Messages app.
Groups can be created in the dedicated Groups section of the app, which provides a separate area to list all ongoing groups as cards; group conversations are also listed in the main Recent section. Groups can be created by picking contacts and choosing a group photo, and once created in Messenger they will sync with Facebook's Messages view on the web and other clients. A minor addition to the chat experience is the Forward button, which can be accessed by tapping & holding an individual message in a conversation.
Facebook Messenger, first launched in 2011, is the company's standalone chat app focused on conversations and photo/location sharing. Version 4.0 is available (still only for iPhone and iPod touch) on the App Store.